CROSS SECTION

in nuclear physics, probability that a given atomic nucleus will exhibit a specific reaction (for example, absorption, scattering, or fission) in relation to a particular species of incident particle. Cross section is expressed in terms of area, and its numerical value is chosen so that, if the bombarding particle hits a circular target of this size perpendicular to its path and centred at a nucleus, the given reaction occurs; and, if it misses the area, the reaction does not occur. The reaction cross section is usually not the same as the geometric cross-sectional area of the nucleus. The unit of reaction cross section is the barn (equal to 10-24 square cm). Values of cross sections for a given nucleus depend on the energy of the bombarding particle and the kind of reaction. Boron, for example, bombarded by neutrons traveling 1,000,000 cm per second (22,500 miles per hour) has a cross section for the neutron-capture reaction of about 120 barns, whereas the cross section increases to about 1,200 barns for neutrons traveling at 100,000 cm per second. These large cross sections mean that boron is a good absorber of neutrons. The metal zirconium, on the other hand, having an absorption cross section of only 0.18 barn for the low-energy neutrons that cause fission in nuclear reactors, is rather transparent to neutrons and is used to clad reactor fuel rods. The reactor fuel uranium-235 absorbs neutrons both by fission and by capture. The fission cross section is 580 barns, whereas the total cross section for neutron absorption (including the capture cross section) is 680 barns.